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From: Kaz Kylheku <847-115-0292@kylheku.com>
Newsgroups: comp.compilers
Subject: Re: Optimization techniques
Date: Fri, 26 Apr 2019 02:26:37 +0000 (UTC)
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Keywords: optimize, design, comment
Posted-Date: 25 Apr 2019 22:34:54 EDT
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On 2019-04-25, David Brown <david.brown@hesbynett.no> wrote:
> It knows that "x * 2 / 2" is "x".  It knows that "x + 1 > x" is true.

Also, we could have it so that multiplication wraps, and yet in the same
breath allow algebraic reductions like this anyway.

Basically it can be so that, if overflow takes place in the abstract
semantics of "x * 2 / 2", the result can be either the truncated
version, or just x, if the algebraic reduction took place.

Algebraic reductions that can change the result can be confined
to evaluation regions similar  to (perhaps tied to) sequence points,
such that if we write:

  int y = x * 2;
  int z = y / 2;

then the calculations are belabored; z is not reduced to x.

Anyway, anyone who wants "x * 2 / 2" to be reduced to "x" through layers
of macrology can always use a more sophisticated macro processor. Parse
it, build a tree, and do the optimization in the preprocessor.

C should serve as a stable "higher level assembly" target for that sort
of thing.
[I take your point but there's an awful lot of macros doing data structure
stuff that frequently give you this kind of code. -John]